In heating, ventilating, and air conditioning (HVAC) systems and house exhaust systems, air flow is typically controlled using resistors to slow down the flow of air to and from different points in a building. When one resistor is adjusted, the pressure level throughout the system will change; any change in the system pressure will affect the flow of air past every other resistor. Thus, adjusting a resistor at one point causes "cross-talk" with resistors at other points.
One of the most complex problems confronted by the HVAC industry is controlling air flow through process rooms, such as the clean rooms used in semiconductor integrated-circuit chip manufacturing, or the medical and biotechnology laboratories kept at below atmospheric pressure to prevent potentially dangerous microbes from blowing out of the laboratories. Some air exits the process room through process equipment and other work stations with fume hoods. A partial vacuum is usually required in such equipment in order to ensure that noxious fumes or dangerous microbes do not leak from the process equipment or fume hoods and thereby endanger personnel working nearby. It is frequently important that a constant partial vacuum be maintained in the process equipment in order to minimize defects in the integrated circuit chips being manufactured. In some process equipment it is important that only a small partial vacuum be maintained.